1. Field of the Invention
The invention relates to a composite film having at least one base layer which predominantly comprises polypropylene, and at least one top layer.
The invention also relates to a process for producing the composite film and to its use as a packaging material.
2. Description of Related Art
Polypropylene films composed of a polypropylene base layer and polypropylene top layers are known in the art. These films typically are characterized as having very good optical properties and good scratch resistance. However, they cannot be processed on automatic packaging machines since they have very poor sealing properties. For sealing, such high temperatures must be applied that pronounced shrinkage occurs in the sealing zone and the packages produced in this manner are not acceptable. Moreover, the imprinting properties and coating properties of oriented polypropylene films require improvement. Although surface treatment of the films (e.g., corona treatment or flame treatment) improves these properties, this improvement frequently decays rapidly with time and storage, or is inadequate.
To eliminate these disadvantages, oriented polypropylene films usually are provided with sealing layers composed of random copolymers or terpolymers of propylene and other .alpha.-olefins, polyethylene or corresponding polymer blends. However, such sealing layers or layers cause a drastic deterioration in the optical properties and, in particular, the gloss is markedly reduced and the opacity increases to an undesired degree. Moreover, sealing properties of the copolymer and terpolymer top layers require improvement.
Coating polypropylene films with dispersions comprising polyvinylidene chloride or various polyacrylates also has been proposed. Although this process also generates sealable films, these films are expensive and have the disadvantage that they cannot be regenerated because the cutting material produced during the film manufacture cannot be recycled due to the coating applied thereto. For economic and ecological reasons, this disadvantage renders these films unacceptable.
EP-A-0,484,817 describes a propylene film with top layers of syndiotactic polypropylene. These films are said to show a balanced property spectrum, in particular good sealing properties and good optical properties, but problems accumulate during stretching of this material. These problems are due to cracking in the syndiotactic top layers or due to delamination of the top layers from the isotactic core layer. The above-mentioned problems are caused, on the one hand, by the poorer stretchability of syndiotactic polypropylene and, on the other hand, by the lower compatibility of syndiotactic and isotactic polypropylene.